Uninterruptible power supply (or UPS) is an emergent power supply device connected between a commercial power supply and a load. The UPS is set to supply the electricity required to power a load in order to ensure the normal operation of the load when the commercial power supply is operating abnormally.
In order to protect important electronic device with efficiency and reliability, UPS has been widely applied to a variety of electronic circuits to ensure the normal operation of the electronic circuits. The current UPS can be roughly classified into three categories: on-line UPS, line-interactive UPS, and off-line UPS.
Referring to FIG. 1(a), a circuit block diagram of a prior art on-line UPS is shown. As shown in FIG. 1(a), the on-line UPS 10 is used to supply electricity to power a load 11 and includes an AC/DC converter 101, a DC/DC converter 102, a DC/AC converter 103, switch elements 104 and 105, a battery module 106, a control circuit 107, a bypass circuit 108, and a charging circuit 109.
The AC/DC converter 101 is used to receive a commercial AC voltage Vin and convert the commercial AC voltage Vin into a DC voltage. The charging circuit 109 is electrically connected to the AC/DC converter 101 for receiving the DC voltage outputted from the AC/DC converter 101 and converting the received DC voltage into a DC voltage required by the battery module 106 so as to charge the battery module 106. The switch elements 104 and 105 as well as the DC/DC converter 102 are under the control of the control circuit 107, which is used to detect the commercial AC voltage Vin and the output voltage of the battery module 106 in order to control the ON/OFF status of the switch elements 104 and 105.
When the internal circuits of the on-line UPS 10 are malfunctioned to supply electricity required by the load 11, the bypass circuit 108 is activated. Under this condition, the control circuit 107 switches the power delivery route from the switch element 104 to the bypass circuit 108 to allow the commercial power supply to provide required electricity to the load 11.
Referring to FIGS. 1(b) and 1(c), a partial circuit diagram of a prior art dual DC-output half-bridge single-phase on-line UPS and a partial circuit diagram of a prior art three-phase on-line UPS are shown. As shown in these diagrams, when the commercial power supply is supplying electricity normally (or called AC mode), the AC/DC converter 101 first rectifies the commercial AC voltage Vin into a rectified DC voltage, and the dual-output step-up DC/DC converter 102 boosts the rectified DC voltage and regulates the boosted DC voltage. Eventually, the DC/AC converter 103 converts the boosted DC voltage into an AC voltage Vout and transmits the AC voltage Vout to the load 11 (as shown in FIG. 1(a)).
The DC/DC converter 102 is set to boost the voltage level of the DC voltage outputted from the AC/DC converter 101 by means of the switching frequency of the internal switch elements S1 and S2. Referring to FIG. 1(d), a control timing diagram of the switch elements S1 and S2 is shown. As can be understood from the timing diagram of FIG. 1(d), the switch elements S1 and S2 are manipulated by way of alternate switching under the AC mode, and the switching frequency of the switch elements S1 and S2 is set to a high frequency. As can be understood from the depiction of the timing diagram, the switch element S2 will be OFF and the switch element S1 will repetitively turn on and off by way of high-frequency switching within the period T1. On the contrary, the switch S1 will be OFF and the switch element S2 will repetitively turn on and off by way of high-frequency switching within the period T2. Therefore, the DC voltage outputted from the battery module 106 to the DC/DC converter 102 can be boosted by the high-frequency alternate switching of the switch elements S1 and S2.
On the other hand, when the commercial power supply can not supply electricity normally (or called DC mode), the DC voltage VBAT outputted from the battery module 106 is boosted by the DC/DC converter 102. Next, the boosted DC voltage of the DC/DC converter 102 is transmitted to DC/AC converter 103 and converted by the DC/AC converter 103 into an output AC voltage Vout. Finally, the output AC voltage Vout is provided to the load 11 through the switch element 104.
Referring again to FIG. 1(d), the switch elements S1 and S1 are also manipulated by way of high-frequency switching. As shown in FIG. 1(d), the switch element S2 will be ON and the switch element S1 will repetitively turn on and off by way of high-frequency switching within the period of T1. On the contrary, the switch element S1 will be ON and the switch element S2 will repetitively turn on and off by way of high-frequency switching within the period of T2. Therefore, the DC voltage outputted from the battery module 106 to the DC/DC converter 102 can be boosted by way of high-frequency alternate switching of the switch elements S1 and S2. However, such high-frequency switching mechanism is feasible on the condition that the regulated predetermined value of the DC voltage across the positive DC side P1 and the negative DC side P2 of the DC/DC converter 102 is higher than the output voltage VBAT of the battery module 106.
Accordingly, when the aforementioned circuit is employed to the application where a lower output DC voltage and a higher battery voltage are required, the aforementioned prior art switching regulation mechanism will become infeasible. For example, when the DC voltage required by the load 11 is 120V and the battery module 106 contains 12 serially-connected batteries each supply a 12V DC voltage, the DC voltage across the DC sides of the DC/DC converter 102 is set to ±220V according to the output voltage of the power supply and the optimized conversion efficiency, and the output voltage VBAT of the battery module 106 is 144V. According to the switching regulation mechanism implied in the timing diagram of FIG. 1(d), the output voltage VBAT (=144V) of the battery module 106 is boosted to ±220V. That is, the output voltage VDC+ at the positive DC side P1 of the DC/DC converter 102 is 220V, and the output voltage VDC− at the negative DC side P2 of the DC/DC converter 102 is −220V. The output voltages VDC+ and VDC− across the positive DC side P1 and the negative DC side P2 of the DC/DC converter 102 are converted by the DC/AC converter 103 into a 120V AC voltage which is to be provided to the load 11.
When the DC voltage required by the load 11 is 220V and the battery module 106 contains 20 serially-connected batteries each supply a 12V DC voltage, the DC voltage across the DC side of the DC/DC converter 102 is set to ±360V according to the output voltage of the power supply and the optimized conversion efficiency, and the output voltage VBAT of the battery module 106 is 240V. According to the high-frequency switching regulation mechanism implied in the timing diagram of FIG. 1(d), the output voltage VBAT (=240V) of the battery module 106 is boosted to ±360V. That is, the output voltage VDC+ at the positive DC side P1 of the DC/DC converter 102 is 360V, and the output voltage VDC− at the negative DC side P2 of the DC/DC converter 102 is −360V. The output voltages VDC+ and VDC− across the positive DC side P1 and the negative DC side P2 of the DC/DC converter 102 are converted by the DC/AC converter 103 into a 220V AC voltage which is to be provided to the load 11.
Because the switch elements S1 and S2 of the DC/DC converter 102 are manipulated by way of high-frequency switching under the DC mode, both of the output voltages VDC+ and VDC− across the positive DC side P1 and the negative DC side P2 of the DC/DC converter 102 will be higher than the output voltage VBAT of the battery module 106. Therefore, when the load 11 required a lower voltage, that is, the regulated predetermined value of the DC voltage across the positive DC side and the negative DC side of the DC/DC converter 102 is required to be lower than the output voltage VBAT of the battery module 106, the prior art UPS can not meet such requirement. For example, when the load 11 requires 120V AC voltage and the battery module 106 contains 20 serially-connected batteries, the DC voltage across the DC side of the DC/DC converter 102 is set to ±220V according to the specified output voltage of the power supply and the optimized conversion efficiency. However, both of the output voltages VDC+ and VDC− of the prior art dual-output step-up DC/DC converter 102 are bound to be higher than the output voltage VBAT of the battery module 106, and thus the output voltage VBAT (=240V) of the battery module 106 has to be boosted to a higher DC voltage level so that the output DC voltage of the DC/DC converter 102 can be converted to 120V AC voltage which is to be provided to the load 11. However, such switching regulation mechanism has worse conversion efficiency, and the electrolytic capacitors located at the output side of the DC/DC converters require higher voltage durability.
More disadvantageously, the switching operation of the switch elements S1 and S2 of the DC/DC converter 102 is achieved by high-frequency pulse-width modulation, and thus its switching loss will be aggravated and the overall power efficiency is deteriorated.
As a result, there is an urgent need to develop a control method for a voltage boosting circuit in order to address the disadvantages lingered in the prior art.